It could morph into a vital part of the rally — one that affects the investments at money management firms across America.
Eaton Vance portfolio manager Michael Allison says tax reform could help drive stocks up by 5 percent to 10 percent more.
"There is some sense of positive movement on tax reform — certainly on the corporate side," Allison said Wednesday on CNBC's "Trading Nation. " "[What] we're spending a lot of time on is really thinking about the changes under the hood."
A corporate tax cut could beef up earnings among companies that currently face a relatively high rate, he said.
It's just one market-moving factor Eaton Vance is watching. The firm manages $405 billion in assets.
Yet there appears to be growing doubt tax reform will be passed — at least at the Federal Reserve.
San Francisco Fed President John Williams acknowledged on Wednesday that he's getting a "little bit discouraged" about whether Capitol Hill can approve one in the next six months or so.
Despite the climbing uncertainty, stocks have been on a tear during the past four weeks.
It's that momentum that's keeping Allison "cautiously optimistic" going into next year. A failure by Washington to approve a tax reform package wouldn't kill the stock market rally, but gains would be more muted, he said.
"There are other factors powering the market, as well, in terms of just a sense of accelerating global growth [and] economic growth," he said.